ACL might be able to do what you want. I have done some interesting things with ACL.
Completely untested:
If you’ve never worked with ACL, the light gray part is match condition and the medium gray is action (row that begins with “Redirect To”)
First entry says “If ingress is port 1, tag or no tag, force egress to port 2 and set vlan id to 60.”
Second entry says “If ingress is port 3 or 4, tagged 60, force egress to port 2 tagged 60.”
Set VLAN ID may not be required in the second entry as the tag already exists.
This way only specific traffic is isolated and vlan 50 is free.